Logarithmic video amplifiers are described by Richard Smith Hughes in his book "Logarithmic Amplification" published by Artech House Incorporated and given the International Standard Book Number 0-89006-182-3. An important application for logarithmic video amplifiers, as identified by Hughes, is that of direction finding, in which a plurality of antennas are mounted onto, say, an aircraft and the position of a radio source, such as a radar transmitter, is determined by comparing the strength of signals received by the antennas. Thus, it is important that all the amplifying circuits exhibit very similar responses to ensure that a processor, arranged to calculate position, receives reliable data.
Radio signals from radar transmitters often consist of pulse modulated carrier waves having a frequency up to 20 GHz, yielding baseband signals (after detection) of up to several hundred MHz. Signals occupying this band may be identified as "video" and relatively fast circuits are required to amplify signals of this type. Devices of this type with particularly wide bandwidths are also identified as wideband logarithmic video amplifiers (WLVA).
A common problem with electronic circuits, and particularly those working with analogue signals, is that their operating characteristics tend to change with variations in operating temperature. This problem is particularly relevant to WLVA's when used in direction finding equipment, because different circuits may be operating at different temperatures and different circuits (although constructed using similar techniques) may behave differently when operating at the same temperature. Techniques are known for compensating for temperature but these are complicated in logarithmic amplifiers which include many amplification stages, as described by Hughes. Temperature dependency becomes even more relevant in video amplifiers which operate right down to dc and are hence dc coupled. As identified at page 74 of Hughes, dc output will drift with temperature at each amplification stage. Page 86 of Hughes goes on to say that each linear amplifier stage may have to be tested over temperature and then set for the best voltage offset, which is identified as being time consuming and costly. Thus, with conventional designs, the designer is often faced with trade-off between operational bandwidth and temperature stability.